Concept for using software/electronics to calibrate the control system for an automatic transmission

ABSTRACT

A software calibration strategy for calibrating solenoid controlled valves and valve systems in an automatic transmission. The strategy includes identifying a characteristic equation for the valve or valve system that is a mathematical relationship between a current applied to the solenoid and the pressure at the output of the valve or valve system. The valve or valve system is coupled to a test stand that depicts the operation of the valve or valve system in the transmission. Current signals are applied to the valve or valve system, and the output pressures are measured to determine coefficients in the equation using a curve fitting function. The coefficients are then stored in a control unit.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of the filing date of U.S.Provisional Application Ser. No. 60/462,225, titled Concept for usingSoftware/Electronics to Calibrate the Control System for an AutomaticTransmission, filed Apr. 11, 2003.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates generally to a software calibrationstrategy for an electrohydraulic transmission control system and, moreparticularly, to a software calibration strategy for calibratingsolenoids, solenoid controlled valve bodies, or solenoid controlledvalve bodies with integrated control units that are used to control anautomatic transmission for a vehicle.

[0004] 2. Discussion of the Related Art

[0005] Automatic vehicle transmissions employ a plurality of gears,synchronizers, brake bands and clutches that are electronicallycontrolled to automatically shift the gears of the vehicle. Typically,solenoid controlled valves are employed within the transmission toprovide hydraulic pressure to control the operation of the variouscomponents and systems therein, such as clutches, brake bands, lubricantflow, hydraulic pressure, etc. The valves are generally proportionalvalves in that the amount of current applied to the solenoid of thevalve determines the hydraulic pressure at the control output of thevalve.

[0006] A solenoid controlled valve includes a coil wound around anarmature and a valve body. The solenoid and the valve body can beattached or separate. The valve body is coupled to a hydraulic inputline, a hydraulic exhaust line and a hydraulic control output line.Hydraulic supply pressure is applied to the input line. The supplypressure applied to the valve is stable, and the control pressure on theoutput line of the valve is set by the position of the armature. When nocurrent is applied to the coil, the control output line is coupleddirectly to the exhaust line so that no output pressure is applied tothe control line. When current is applied to the solenoid, the armaturemoves so that the hydraulic supply pressure is directed to the controlline. The amount of current applied to the coil sets the force on thearmature, and thus its ability to maintain its regulated outputpressure. The valve is regulating in that if the load on the controlline changes, the armature moves in response thereto to provide the sameoutput pressure for that current.

[0007] Proportional solenoid controlled valves need to be calibrated sothat they provide the desired control pressure relative to the inputcurrent. If the control valves in the transmission are not properlycalibrated, then the performance of the transmission is reduced by avariety of factors, such as lower fuel economy, hard shifts, etc.Typically, the valve is calibrated by mechanical devices when the valveis manufactured. One known mechanical device is a threaded mechanismthat increases or decreases a spring force applied to the armature. Asecond mechanical technique for calibrating a solenoid or solenoidpiloted valve is to selectively adjust the working air gap between thearmature and pole piece of the solenoid, which adjusts the magneticforce level of the device.

[0008] The known mechanical techniques of calibrating a solenoid valveare typically costly because the calibration components are expensive tomanufacture and the calibration process is time consuming. Further, theequipment required to perform the calibration is expensive and hasinherent accuracy limitations. Additionally, the solenoid or solenoidpiloted valve is not calibrated to other system components, such aselectronics, other valves, leakage, etc, that may affect the valveperformance when it is employed in the system.

SUMMARY OF THE INVENTION

[0009] In accordance with the teachings of the present invention, asoftware calibration strategy is disclosed that has particularapplication for calibrating solenoid controlled valves andelectrohydraulic control systems employed in an automatic transmission.The calibration strategy includes identifying a characteristic equationthat is unique for a particular class of valves or valve systems, andidentifies a mathematical relationship between the current applied tothe solenoid and the pressure, or other force such as fluid flow, at theoutput of the valve or valve system. The characteristic equation willinclude several coefficients that are unique for each separate valve orvalve system in the class.

[0010] The valve or valve system is coupled to a test stand thatsimulates the operation of the valve or valve system in thetransmission. Control current signals are applied to a solenoid in thevalve or valve system and the output pressure (or flow) of the valve orvalve system is measured. Once the current and corresponding pressure isknown for several points, the coefficients for the particular valve orvalve system can be determined by using a curve fitting technique, suchas a least squares regression, on the characteristic equation. Once thecoefficients are identified, they are stored in a flash memory of atransmission control unit (TCU) that is typically integrated into atransmission control module. The coefficients are then used incombination with the characteristic equation, which is part of thevehicle software stored in a TCU memory, to create a unique, highprecision, continuous mathematical representation of the valve or valvesystem performance characteristic.

[0011] Additional advantages and features of the present invention willbecome apparent from the following description and appended claims,taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a flow chart diagram showing a process of calibratingvalve and/or valve systems, according to an embodiment of the presentinvention;

[0013]FIG. 2 is a graph with commanded pressure on the horizontal axisand actual pressure on the vertical axis showing the pre/postcalibration relationship between the actual measured pressure and thecommanded or desired pressure for a large population of proportionalpressure control devices; and

[0014]FIG. 3 is a graph with current on the vertical axis and pressureon the horizontal axis showing the relationship between current andhydraulic pressure for a clutch pressure control valve in an automatictransmission.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0015] The following discussion of the embodiments of the inventiondirected to a software calibration strategy for an automatictransmission is merely exemplary in nature, and is in no way intended tolimit the invention or its applications or uses.

[0016]FIG. 1 is a block diagram 70 showing a process for calibratingvalve systems and valves in an automatic transmission, according to theinvention. The calibration strategy of the invention employs a teststand that simulates the operation of the transmission. An algorithm isemployed to simulate the operation of the transmission to determine theactual pressures or flows at certain operating conditions.

[0017] Each class of valve and/or valve system can be represented by acharacteristic equation, the complexity of which is a function of thedesired accuracy of the fitting function and the degree of non-linearityand number of inflection points of the actual performancecharacteristic. The general form of the equation is requiredinput=function (desired output), where the desired output is pressure,flow, etc. and the required input is solenoid current.

[0018] The characteristic equation for each valve or valve system ispart of the transmission control software code, which resides in a TCUmemory. This equation does not change from part to part. A typicalcharacteristic equation for a proportional control solenoid is providedin equation (1). Unique coefficients for the characteristic equation,identified as C₁, C₂, C₃, C₄ and C₅ for the characteristic equation in(1), are created by using a curve fitting technique, for exampleleast-squares regression, using actual measured data obtained during thefinal test of the valve and/or valve system as an input to the fittingalgorithm. $\begin{matrix}{{i(P)} = {C_{1} + \frac{C_{2}}{1 + P} + {C_{3} \cdot P} + {C_{4} \cdot P^{2}} + \frac{C_{5}}{P^{3} + 0.0001}}} & (1)\end{matrix}$

[0019] The coefficients are flashed to the TCU memory at the end of thefinal production test procedure, resulting in a unique equationdescribing the performance characteristic of the valve and/or valvesystem. It is important to note that because the calibration isperformed on the complete system, rather than on the individualcomponents, the overall system variation due to downstream/upstreamvalve geometry, spring loads, leakage, solenoids, electronics, etc. issignificantly reduced.

[0020]FIG. 2 shows pre and post calibration results obtained from alarge population of proportional pressure control devices in a knownpressure control system. Upper and lower bounds are shown for thepopulation based on a +/−5 sigma normal distribution. The graph lines 80and 82 represent a +/−5 sigma distribution before the calibration, andthe graph lines 84 and 86 represent a +/−5 sigma distribution after thecalibration.

[0021] The first step of the process at box 72 includes identifying thecharacteristic equation for a particular valve or valve system, whichbecomes part of the transmission control software and resides in TCUmemory. The next step of the process at box 74 includes measuring theoutput pressure or flow of the valve or valve system for a series ofinput currents applied to the solenoids discussed above. The number oftest currents required depends on the number of variables in thecharacteristic equation and the degree of non-linearity and number ofinflection points in the characteristic equation, and the desiredaccuracy of the fit. In a typical embodiment, the number of testcurrents may be in the range of 5-8 test currents.

[0022] The next step of the process includes determining thecoefficients as represented by box 76. In one embodiment, thecoefficients are determined by a regression technique, for example, aleast-squares method, that calculates the coefficients for thecharacteristic equation that result in a best fit to the measured dataobtained in the previous step at the box 74. The result is a unique,continuous mathematical relationship between the current applied to thesolenoid and the pressure (or flow, position, etc.) at the output of theelectrohydraulic system.

[0023] Equation (2) below shows a typical characteristic equation, withcalculated coefficients, for a proportional pressure control system.Once the coefficients are determined by the least-square fittingfunction or otherwise, the coefficients are stored in a flash memory ofthe electrohydraulic system, as represented by box 78. Thecharacteristic equation resides in the TCU memory and is imbedded in thetransmission control software. Note that the characteristic equationneed not be flashed at this time because it is not unique to each partwithin a given class. Only the coefficients are unique. $\begin{matrix}{{i(P)} = {0.376 - \frac{.242}{1 + P} + {0.059 \cdot P} + {1.681 \cdot 10^{- 5} \cdot P^{2}} - \frac{7.328 \cdot 10^{- 9}}{P^{3} + 0.0001}}} & (2)\end{matrix}$

[0024]FIG. 3 is a graph with current on the vertical axis and pressureon the horizontal axis showing the general current to pressurerelationship for the characteristic equation (1).

[0025] The foregoing discussion discloses and describes merely exemplaryembodiments of the present invention. One skilled in the art willreadily recognize from such discussion and from the accompanyingdrawings and claims that various changes, modifications and variationscan be made therein without departing from the spirit and scope of theinvention as defined in the following claims.

What is claimed is:
 1. A method of calibrating an electrohydrauliccontrol system that provides an output response in response to an inputcurrent, said method comprising: identifying a characteristic equationof the electrohydraulic system, said characteristic equation including aplurality of coefficients; coupling the electrohydraulic system to atest stand; applying a plurality of currents to the electrohydraulicsystem; measuring the output response of the electrohydraulic system foreach of the plurality of currents; and identifying the coefficients inthe characteristic equation from the output response measurements. 2.The method according to claim 1 wherein identifying the coefficients inthe characteristic equation from the output response measurementsincludes employing a curve fitting function.
 3. The method according toclaim 2 wherein identifying the coefficients in the characteristicequation from the output response measurements includes employing aleast squares fitting function.
 4. The method according to claim 1further comprising flashing the coefficients in a memory.
 5. The methodaccording to claim 1 further comprising hard-coding the characteristicequation into control software.
 6. The method according to claim 1wherein the electrohydraulic system includes a proportional solenoid anda hydraulic valve, wherein applying a plurality of currents to theelectrohydraulic system includes applying a plurality of currents to theproportional solenoid.
 7. The method according to claim 1 wherein theelectrohydraulic system is employed in an automatic transmission.
 8. Themethod according to claim 7 wherein the electrohydraulic system isemployed in a pressure regulation system or a flow regulation systemused for controlling functions in the automatic transmission.
 9. Themethod according to claim 1 wherein the electrohydraulic system includesan integrated transmission control unit (TCU).
 10. The method accordingto claim 1 wherein the output response is selected from the groupconsisting of pressure and fluid flow.
 11. The method according to claim1 wherein applying a plurality of currents to the electrohydraulicsystem includes applying a plurality of different currents.
 12. A methodof calibrating an electrohydraulic system employed in an automatictransmission, said electrohydraulic system providing an output responsein response to an input current, wherein the electrohydraulic systemincludes a proportional solenoid, a hydraulic valve, and solenoid driveelectronics, said method comprising: identifying a characteristicequation of the electrohydraulic system, said characteristic equationincluding a plurality of coefficients; coupling the electrohydraulicsystem to a test stand; applying a plurality of currents to the solenoidcontrolling the valve; measuring the output response of theelectrohydraulic system for each current; identifying the coefficientsof the characteristic equation from the output response measurements,wherein identifying the coefficients in the characteristic equation fromthe output response measurements includes employing a curve fittingfunction; and storing the coefficients in an on-board memory.
 13. Themethod according to claim 12 wherein the electrohydraulic system isemployed in a pressure regulation system or a flow regulation systemused for controlling functions in the automatic transmission.
 14. Themethod according to claim 12 wherein identifying the coefficients in thecharacteristic equation from the output response measurements includesemploying a least squares fitting function.
 15. The method according toclaim 12 wherein the output response is selected from the groupconsisting of pressure and fluid flow.
 16. An electrohydraulic systemcomprising: a device for determining a characteristic equation of theelectrohydraulic system, said characteristic equation including aplurality of coefficients; a device for applying a plurality of currentsto a proportional solenoid in the system; a device for measuring anoutput response of the electrohydraulic system for each current; and adevice for determining the coefficients in the characteristic equationfrom the output response measurement.
 17. The system according to claim16 wherein the device that determines the coefficients in thecharacteristic equation from the output response measurement employs acurve fitting function.
 18. The system according to claim 16 furthercomprising a memory for storing the coefficients.
 19. The methodaccording to claim 16 wherein the electrohydraulic system is employed inan automatic transmission.
 20. The system according to claim 19 whereinthe electrohydraulic system is employed in a pressure regulation systemor a flow regulation system used for controlling functions in theautomatic transmission.
 21. The system according to claim 16 wherein theoutput response is selected from the group consisting of pressure andfluid flow.